
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>类选择器</title>
    <style>
        /*
            类选择器是为一类状态声明的规则
        */
        .text-center{
            text-align: center;
        }

        /*将类选择器指定为具体标签*/
        .help-block{
            background: red;
        }
        span.help-block{
            font-size: 12px;
            color:#aaa;
            background: none;
        }

        /*
            id选择器
        */
        #container{
            background: red;
        }
    </style>
</head>
<body>
    <h1 class="text-center">houdunren.com</h1>
    <h2 class="text-center">hdcms.com</h2>
    <span class="help-block">感谢访问后盾人</span>

    <h1 id="container">houdunren.com</h1>
</body>
</html>
